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Two years ago, several research vessels shipped out to the North and the South poles to assemble a census of creatures living under the ice. One of the most surprising results was a discovery that 235 identical species lived on opposite sides of the world but were undocumented anywhere else. It's easy to understand how massive humpbacks can swim from Arctic to Antarctic waters, but most of the miniature worms, snails and crustaceans on the researchers' list are no bigger than grains of rice. How could tiny creatures adapted for the frigid waters travel 9,500 kilometers through warmer climes to reach the opposite pole?

Under the microscope, these invertebrates sometimes look like shredded plastic bags or shrimp with bullhorns. It's unclear how they could cross a swimming pool, let alone the globe. So, their "bipolarity" poses a 160-year mystery of the ocean—one that has only grown with time. "If bipolar species are as common as our initial list suggests, it really means we don't appreciate the mechanisms that are important for connectivity in the ocean as well as we thought," says Russ Hopcroft, project leader of the Arctic portion of the Consortium for Ocean Leadership's Census of Marine Life.

The discovery of bipolar species dates back to the 1840s expeditions of Victorian explorer James Clark Ross and his two heavy-duty battle cruisers, the HMS Erebus and Terror. During missions to map the North and South poles, he collected samples of marine flora and fauna that looked remarkably similar. He theorized that somehow these tiny species had been able to survive not only the icy waters that would eventually sink his ships, but also a journey halfway around the planet.

Since then, skeptics have bickered about the evidence. Some complained that the underwater life specimens were misidentified or appeared too different. But in 2000 Kate Darling, an oceanographer at University of Edinburgh in Scotland, settled any debate. In the northern and southern subpolar waters off Iceland and the Falkland Islands, respectively, Darling collected foraminifera, single-celled ocean drifters that look like prickly gobs of bubblegum. When she sequenced the ribosomal DNA from three species—Globigerina bulloides, Turborotalia quinqueloba and Neogloboquadrina pachyderma—she found the genes to be so similar that, she says "they must be mixing, maybe even now." (Rather appropriately, she collected her samples on board the British research vessel named after her predecessor, the James Clark Ross.)

The same year that Darling published her results, thousands of marine biologists banded together to map the creatures of the oceans for the census. The decade long, $650-million campaign launched hundreds of research voyages around the globe, with dozens heading to the poles. Nothing of its scope had ever been undertaken before.

"It gave us the ability to start seeing larger-scale patterns than any of us can see individually in our own little backyards," Hopcroft says. Last year, when the Arctic and Antarctic surveyors pooled their data, the mystery of bipolarity ballooned to 235 species. The question resurfaced: How do the same types of creatures span both poles?

Some scientists and naturalists, including Charles Darwin, have hypothesized that species migrated over thousands of years when average ocean temperatures were much colder, probably at some point in geologic time between the Tertiary period and the last ice age 18,000 years ago. Darling's data, however, puts a dent in that theory. The minuscule genetic differences in her "bugs," as she calls them, suggest that the species mingled far more recently.

Today, most scientists think the species travel a deepwater conveyor belt called the thermohaline circulation, the ocean-wide phenomenon responsible for currents such as the Atlantic's Gulf Stream. Because cold water at both poles changes salinity and sinks as it spreads, it forms discrete submarine rivers that descend at the equator and resurface at opposite ends of the planet. Along the way, temperatures only waver between 2 to 4 degrees Celsius, consistent enough for most polar dwellers to survive. The creatures themselves make the journey from one pole to its antipode suspended as larvae or eggs, or as live adults, reproducing over generations on their 9,500-kilometer trek before arriving 400 to 600 years later. To return to their pole of origin it could take another 1,600 years because of prevailing currents.

Besides the enormous travel time, the theory partially refutes bipolarity: It suggests that species may live outside of polar regions but we just haven't found them yet. "It's the awkwardness of how you define something. Is the definition functional or based on lack of data? We know so little about the deep layers of the ocean compared to the surface," Hopcroft says.

The list of bipolars is tentative for another reason: biologists identified most of the species based on morphology, or form and structure. Species that live in similar environments often look identical but can be genetic strangers. "Until you do the genetics you can't know that they're bipolar for sure," Darling says.

In the past year, a census team has been scouring marine collections to find specimens from both poles that were preserved in a grade of ethanol pure enough to enable taking DNA samples from the organisms. (The vast majority has been stored in formaldehyde and cannot be genotyped.) To investigate the specimens' biopolarity, they have so far sequenced hundreds of samples of mitochondrial DNA known as the bar-coding gene. They hope to finish their search by the time the Census of Marine Life is published in October.

"I think our results are really going to revise the paradigm about the connection between the poles," Hopcroft says. Each life-form has likely made a unique journey, but viewed together they will reveal a world intertwined, even at its ends.